High demands are placed today on photosensitive compositions used, in particular, for high-performance printing plates. Essentially two avenues are taken to improve the properties of photosensitive compositions and thus, of the corresponding printing plates. One deals with improving the properties of the photosensitive components of the compositions (negative diazo-resins, photoinitiators, photopolymers, etc.), the other with the discovery of new polymer compounds (binders) which are intended to control the physical and mechanical properties of the photosensitive layers. The latter course is of special significance to printing plates, because the behavior in the development and printing process (such as development capability, ink acceptance, scratch resistance, plate life) is critically influenced by the polymer binders selected. The shelf life and photosensitivity of printing plate materials are also influenced by such polymer compounds.
The polymer binders have various structural elements to accomplish the many requirements, and these may have differing effect on individual properties. Thus, hydrophilic structural elements such as carboxyl groups, hydroxyl groups, etc., generally promote a good developability of the photosensitive compositions in aqueous-alkaline developers and contribute to adequate adhesion to polar substrates. On the other hand, hydrophobic structural elements hinder the developability in said developers, but ensure good acceptance of the ink in the printing process, which is absolutely essential for printing plates.
Given the broad spectrum of requirements on polymer binders, there has been much work for many years on the synthesis and optimization of the use of these substances for photosensitive compositions, see for example, H. Baumann and H. J. Timpe, Chemical Aspects of Offset Printing, J. prakt. Chem./Chemiker-Zeitung, Vol. 336, pgs. 377-89 (1994).
U.S. Pat. Nos. 4,511,640 and 4,618,562 and U.S. Pat. No. 4,731,316, describe binder systems which consist of mixtures of various polymers, possessing different hydrophilic/hydrophobic properties. Yet such mixtures have drawbacks in that separations of the mixture often occur when forming layers, on account of incompatibilities among the substances. Furthermore, separations of the hydrophobic polymers in the development step when processing the printing plates, may result in silting in the development machines and redeposition of dispersed elements of the layer on the surface of the photosensitive material.
Moreover, various copolymers of less hydrophilic monomers, such as styrene, acrylate, methacrylate, etc., with hydrophilic comonomers have been described. Examples of such comonomers are semi-esters of maleic acid (Canadian Patent 1 172 492 and U.S. Pat. No. 4,687,727) itaconic acid (U.S. Pat. No. 5,260,161), and acrylic acid or methacrylic acid (European Patent EP-A-487 343, U.S. Pat. Nos. 4,304,832 and 4,123,276). The disadvantage of such polymers is the very narrow margin of applicability for important properties such as film adhesion, developability, ink acceptance, and plate life. Due to the production process, one can hardly avoid variations in the polymer compositions, resulting in unacceptable fluctuations in the properties of the plates.
U.S. Pat. No. 4,177,073 describes a photosensitive compositions in which the binder is a reaction product of cellulose esters with cyclical, intramolecular acid anhydrides of dicarboxylic acids. However, these binders have too little oleophilia for use in printing plate formulations. Furthermore, their plate life does not meet the demands on a modern printing plate.
Another group of binders, acetals of aliphatic aldehydes, with unsubstituted lower alkyl group and vinyl alcohol/vinyl acetate copolymers, have been described (U.S. Pat. No. 2,179,051 and U.S. Pat. No. 4,665,124) . But such binders cause problems during the developing of printing plates, owing to the insufficient proportion of hydrophilic groups in the polymer. As an improvement, partially acetalized vinyl alcohol/vinyl acetate copolymers have been provided with hydrophilic or alkaline-soluble groups by special reactions. U.S. Pat. No. 4,940,646 teach aldehyde containing hydroxyl groups that are used for the acetalization, besides aldehydes with hydrophobic groups (e.g., alkyl or aryl groups). Yet this structural change does not result in a distinct improvement in developability.
In order to improve developability, sulfonyl urethane groups were introduced into polyvinyl acetals in U.S. Pat. Nos. 3,372,105 and 3,732,106 and 4,387,151. However, the low acidity of these groups requires developers with a large amount of solvent. Furthermore, the microelements of the resulting printing plate have very poor adhesion and are easily abraded in the printing process.
UK Patent GB 1 396 355 and U.S. Pat. No. 3,847,614, describe binders which can be produced by acetalization of saponified copolymers of vinyl acetate and a carboxyl group-carrying monomer, such as crotonic acid. However, this type of binder results in systems with poor photosensitivity and low plate life when used for printing forms. Moreover, such compositions are poorly developable in aqueous-alkaline developers, because the acid number of the binder is greatly decreased by chemical reactions of the carboxyl group during the saponification and/or acetalization process.
U.S. Pat. Nos. 5,045,429 and 4,681,245, describe introducing carboxyl groups by reacting acetals, which have been separately produced from aliphatic aldehydes and polyvinyl alcohol, with intramolecular cyclical acid anhydrides of dicarboxylic acids. However, the synthesis is uneconomical, for in addition to the acetalization, there is the reaction with the acid anhydrides, which is only possible in aprotic solvents. Furthermore, the photosensitivity of the compositions prepared from this binder is too low.
U.S. Pat. No. 4,741,985 describes binders produced in a three-stage synthesis, starting with polyvinyl alcohol: .sup.(1) acetalization with aliphatic aldehydes, .sup.(2) reaction with intramolecular cyclical acid anhydrides of dicarboxylic acids, and .sup.(3) partial esterification of the carboxyl groups with substituted alkyl halides. Despite the high expense in synthesis of the binder, the photosensitivity of the film made is inadequate.
The expense of a multi-stage synthesis can be avoided if, as described in U.S. Pat. No. 5,219,699, U.S. Pat. Nos. 4,652,604, 4,895,788, 4,940,646 and 5,169,897, polyvinyl alcohol is reacted with aliphatic aldehydes and carboxyl-containing aliphatic or carboxyl-containing aromatic aldehydes. However, the photosensitive compositions prepared from this have an unfavorable relationship in terms of developability and sensitivity.
Mixtures of polyvinyl acetals and organic polymers, which possess an acid number greater than 71 mg KOH/g have also been described in U.S. Pat. No. 5,143,813. But here, again, incompatibilities and, thus, separation of the mixture can occur during lamination or drying of the photosensitive compositions.
Polymers containing urethane resins have been described as binders for photosensitive compositions U.S. Pat. Nos. 5,206,113, 5,238,772, 5,187,040 and 5,176,985 and U.S. Pat. Nos. 4,983,491, 4,950,582, and 4,877,711). These polyurethane resins contain functional groups (--COOH, --SO.sub.2 NHCOO--, --CONHSO.sub.2 NH--) having acidic hydrogen atoms, whose pK.sub.a values in water are not larger than 7. However, functionalization with such hydrophilic groups requires a very large synthesis expense and results in high manufacturing cost.
Despite the intensive research in the field of photosensitive compositions for printing plates, all existing compositions can stand improvement, especially in terms of their developability and sensitivity to printshop chemicals. Again, many of the compositions either have a large number of sometimes expensive ingredients, or the production of their main ingredients involves a large synthesis expense, both of which factors prevent economical applications of the binder composition.
It is therefore an object of the present invention to prepare binders for photosensitive compositions whose production entails the smallest number of synthesis steps possible, as compared to the binders described in the prior art, and which, in photosensitive compositions, nevertheless exhibit the same or (in certain areas) improved physical properties. In particular, these improved properties include, inter-alia, improved ink acceptance and/or a larger print run of the corresponding printing plates ought to be achieved, as compared to the compositions described in the prior art. Yet furthermore, a good developability in developers containing small amounts of organic solvents, salts, and wetting agents, as well as a high photosensitivity and good image resolution, should be retained. Finally, the compositions should be compatible with the other products used in the processing of printing plates.
A further object of the invention is to provide photosensitive compositions containing these binders.
Yet, a further object of the invention is provide printing plates employing such photosensitive compositions.
Other objects and advantages of the invention will become apparent from the following description of embodiments.